Circuit Switching and Packet Switching

Home , Circuit switching, Frame (networking), Node (networking), Packet switching, Throughput, Virtual circuit

Switched Network o Physically separate path for each pair of communicating end systems o Using switches we can reduce the number of paths

Point-to-point network Switched network Switched Network

• Stations • Switch nodes (N) • Nodes can be access points • Transmission links • Nodal degree • Number of links

Max. number of links: N(N-1)/2

Example: Design a network with 5 nodes 10 stations average nodal degree of 2. How many links are required? What will be the nodal degree for a full mesh? Example

3 Communications Network o a collection of nodes and links o nodes may connect to other nodes only, or to stations and other nodes o network is usually partially connected (not a full mesh) n some redundant connections are desirable o switching technologies n circuit switching n packet switching Circuit Switching o uses a dedicated path between two stations n path is reserved for the single pair of end users o has three phases n circuit establish n data transfer n circuit disconnect o inefficient n channel capacity dedicated for duration of connection n if no data, capacity wasted n inefficient use of the path if there is bursty traffic o set up (connection) takes time – setup time o once connected, transfer is transparent Public Circuit Switched Network

Thousands of users May be connected directly to the end-office Dallas, TX Rohnert Park, CA caller number: 403-xxxx 214 707 214-403-2211

Exchange Code NPA – NXX - XXXX 663-xxxx Area Code Subscriber Line Number

Check out the Geographical NPAs Plan. Direct link to the subscriber 22xx

o North American Numbering Plan (NPAN) o International Direct Distance Dialing (IDDD) n Each country has a country code n To access the country code you need a notification code (0 or 011) Called user number: 707-663-2211 Circuit Establishment

- Once the circuit is setup the transmission is transparent à no transmission delay and no delay variations

Santa Rosa

San Francisco Circuit Switching Inefficiency o Assume set-up time is 150 msec and the data length is 1000 bytes transmitting at 64Kbps. n Only takes 125 msec to transmit the data! o Not efficient when data occurs in bursts separated by idle periods

Bursty data: Event Timing o Assume R=2.5Gbps o N=4 (intermediate nodes) o Message length L=1000Kbit o Control Message length L’ = 10 bit o Data Transmission Delay = L/R (sec) o Control Message Transmission Delay = L’/R (sec) Circuit Switch Elements o NI allows CU connecting to digital or analog lines o CU sets up the

switch fabric (SF) SF o SF is the hardware that actually causes switching

NI Switch Fabric using Crosspoints

This is a non-blocking Switch Blocking or Non-blocking o non-blocking network n permits all stations to connect at once n used for some data connections o blocking network n may be unable to connect stations because all paths are in use n used on voice systems o non-blocking is cheaper n not everyone can make a call at the same time

Switching technologies: • Space Division Switching • Time Division Switching Space Division Switch - Non-blocking o NxN crosspoints o For 10^6 users, a crossbar would require 10^12 crosspoints (N^2) Multiple Stage Space Switch -

n x k k x n N x N n n n x k k x n

n x k k x n N x N n n n x k k (N/n x N/n) k x n second stages! N = 16 n = 4 16x16 Rectangular Array: 256 crosspoints k = 2 Three Stage Switch: 96 crosspoints Blocked Calls are possible...

n x k k x n N x N n n n x k k x n

n x k k x n N x N n n n x k k (N/n x N/n) k x n second stages! N = 16 n = 4 2 lines blocked. k = 2 N = 10 n = 5 3 Stage Space Division Switch k = 2

3 lines blocked. n x k (4x2) … also called Clos Network o if k = 2n-1 or greater nonblocking switch o If k < 2n-1, blocking can occur o ….. the higher k value the more crosspoints are required for the crossbar switch! (for each additional k we add n more crosspoints!) o Example: suppose that 1000 users using 10 input per input module. How many second stages are required in order to have a non- blocking switch? Example - Solution

n N=1000, n=10; o In the case of a full 1000 x 1000 crossbar switch, no blocking n There are N/n = 1000/10 occurs but 1,000,000 (a million) = 100 switches at the crosspoints are required. first and third stages. n At the first stage, there are 10 x k and at the third o For n=10 and k=19, each switch stage at the first stage is a 10 x 19 crossbar which requires 190 n There are k switches in crosspoints and there are 100 the second stage such switches. n The second stage will o Same for the third stage. have k switches of size 100 x 100. o So the first and third stages use 2x190x100=38,000 crosspoints n If k = 2n-1=19, then the altogether. resulting switch will be nonblocking. o The second stage consists of k=19 crossbars each of size 100 x 100 n If k < 19, then blocking because N/n=1000/10 = 100. can occur. o So the second stage uses 190,000 crosspoints. o Altogether, the Clos construction uses 228,000 crosspoints

http://people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture11/switching/clos_network/clos_network.html Power Consumption In Modular Design

n x k k x n N x N n n n x k k x n

n x k k x n N x N n n n x k k (N/n x N/n) k x n second stages! Power Consumption In Modular Design - Power Efficient Architecture

n x k k x n N x N OFF n n n x k k x n

n x k k x n N x N n n

k (N/n x N/n) n x k OFFk x n second stages! Example (Remember)

n x k k x n N x N n n n x k k x n

n x k k x n N x N n n n x k k (N/n x N/n) k x n second stages! Time Division Switching o modern digital systems use intelligent control of space & time division elements o use digital time division techniques n set up and maintain virtual circuits n partition low speed bit stream into pieces that share higher speed stream n individual pieces manipulated by control logic to flow from input to output Remember: This is circuit switching which uses time slots (TDM)!

http://archvlsi.ics.forth.gr/~kateveni/534/04a/s21_ts.html Packet Switching o circuit switching was designed for voice o packet switching was designed for data n transmitted in small packets o packets contains user data and control info n user data may be part of a larger message n control info includes routing (addressing) info o packets are received, stored briefly (buffered) and past on to the next node n packets are queued Packet Switching Advantages o line efficiency n single link shared by many packets over time n packets queued and transmitted as fast as possible o data rate conversion n stations connect to local nodes at their own speed n nodes buffer data if required to equalize rates o packets accepted even when network is busy o priorities can be used Data Rate Conversion

eecs122, walrand Switching Techniques o The station breaks long message into packets o Packets are sent one at a time to the network o Packets can be handled in two ways n datagram n virtual circuit Datagram Diagram o No pre-planned route à fast (no circuit) o Each packet can pass through a separate path o Reassembly is required o Packets may experience jitter (delay variation) o Network can provide error control o More flexible (more primitive) o More reliable (if a node fails circuit fails)

Virtual Circuit Diagram o call setup phase is required o Fixed route (circuit switching) o Each packet has VC ID o No routing decisions at the intermediate nodes --> fast delivery Event Timing (With ACK) Due to packet Event Timing queue!

No call setup

Each packet is routed in dependently à higher jitter and possibly delay per node X Y Packet Size and a b N=2 Delay Two Hops!

K = Message_length(bit) e.g., entire message P = Packet _length(bit) R = Line_ transmission _ rate(bps) N = # switch _ nodes _in _ the_ path

Dcircuit _ switch = K / R P K ! P D = (N +1)+ packet _ switch R R

K If the whole message is Dmessage_ switch = (N +1) broken into multiple R smaller packets. Note: we are ignoring the overhead In each case the entire and setup delay and propagation delay message/packet must be received before The transmission delay retransmission! for the first packet Example:

o Assume the message is 10^6 bits long K =1000000bit o Transmission rate is P = 2000bit 50Kbps R = 50000bps o Four switches in the N = 4

path Dcircuit _ switch = K / R o Packet size is 2000bit =1000000 / 50000 = 20 sec o Calculate the delay for P K − P K PN D = (N +1) + = (1+ ) packet switched packet _ switch R R R K network and circuit = 2000(5) / 50000 + (1000000 − 2000) / 50000 = 20.16 sec switched network o Neglect call setup and overhead

Applet: Note if P<

o Assume the message is 10^6 bits long K =1000000bit The Throughput = o Transmission rate is P = 2000bit Total number of bits transmitted 50Kbps R = 50000bps ------o Four switches in the N = 4 Total Delay

overhead Total number of useful bits transmitted ------Total Delay

Applet:

http://www.slideshare.net/Convergent_Technology/next-generation-networks-8193677 Circuit vs. Packet Switching o performance depends on various delays n propagation delay n transmission time n node delay n transparency n amount of overhead X.25 o ITU-T standard for interface between host and packet switched network o almost universal on packet switched networks and packet switching in ISDN o defines three layers n Physical n Link n Packet X.25 - Physical o interface between station node link o two ends are distinct n Data Terminal Equipment DTE (user equipment) n Data Circuit-terminating Equipment DCE (node) o physical layer specification is X.21 o can substitute alternative such as EIA-232 X.25 - Link o Link Access Protocol Balanced (LAPB) n Subset of HDLC (described later) o Provides reliable transfer of data over link o Sends a sequence of frames X.25 - Packet o provides a logical connections (virtual circuit) between subscribers o all data in this connection form a single stream between the end stations o established on demand o termed external virtual circuits X.25 Use of Virtual Circuits User Data and X.25 Protocol Control Information

Link Access Protocol Balanced Issues with X.25 o key features include: n call control packets, in-band signaling n multiplexing of virtual circuits at layer 3 n layers 2 and 3 include flow and error control, hence, have considerable overhead o no hop by hop error or flow control (rather than end-to-end) – hop-by-hop ACK o not appropriate for modern fast digital systems requiring high reliability Frame Relay o designed to eliminate most X.25 overhead o has large installed base o key differences: n call control carried in separate logical connection n multiplexing and switching at layer 2 n no hop by hop error or flow control n Hence, end to end flow and error control (if used) are done by higher layer o a single user data frame is sent from source to destination and higher layer ACK sent back Advantages and Disadvantages o lost link by link error and flow control o increased reliability means less an issue o streamlined communications process n lower delay n higher throughput o frame relay can be used for access speeds up to and over 2Mbps Protocol Architecture LAPF Functionality o LAPF (Link Access Procedure for Frame Mode Bearer Services) defined in Q.922 o only core functionality used: n frame delimiting, alignment and transparency n frame mux and demux using addressing field n ensure frame is integral number of octets n ensure frame is neither too long nor short n detection of transmission errors n congestion control functions o form sub-layer of data link layer n data transfer between subscribers only Frame Relay Data Link Connections o logical connection between subscribers o data transferred over them o not protected by flow or error control o uses separate connection for call control o overall results in significantly less work in network User Data Transfer o only have one frame type which n carries user data o no control frames means n no inband signaling n no sequence numbers o flag and FCS function as in HDLC o address field carries DLCI o DLCI (Data Link Connection Identifier) has local significance only Summary o circuit verses packet switching network approaches o X.25 o frame relay References o http://people.seas.harvard.edu/~jones/ cscie129/nu_lectures/lecture11/switching/ clos_network/clos_network.html o Switch design http://www2.cs.uh.edu/~johnsson/ cosc6365_08/Lecture09_S.pdf o Circuit-Switched Coherence www.eecg.toronto.edu/~enright/circuit- switched-coherence.ppt